Automatic powershiftable transmissions for vehicles, known as automatic transmissions for shod, usually have as the starting element a hydrodynamic torque converter. Predominantly, these transmissions are designed as range-change transmissions which comprise several planetary gearsets for obtaining a number of gears or gear steps, which are usually engaged by means of hydraulic shifting elements such as disk clutches or disk brakes.
To function properly a transmission needs an effective oil supply for lubrication and cooling, as well as particular filling quantities and oil pressures for shifting the hydraulic components. For this, as a rule a hydraulic pump coupled to a transmission input shaft and driven by the combustion engine is provided and which provides the necessary oil pressure by way of a regulated, valve-controlled hydraulic system.
To reduce the emissions of harmful substances, fuel consumption and noise levels it is also desirable, particularly in urban traffic, to have a so-termed start-stop operating mode in which, depending on the situation and possibilities, when the vehicle is at rest, for example at road junctions with traffic lights, the combustion engine should be switched off. After a subsequent engine start the transmission must be ready again for the transmission of torque as soon as possible. However, in vehicles with torque converter automatic transmissions this is not necessarily straightforward.
Since the hydraulic pump is driven by the combustion engine, pressurized oil is only available when the combustion engine is running. In contrast, when the combustion engine is switched off the hydraulic system loses pressure, whereupon the transmission control system usually disengages the current gear and shifts the transmission to neutral. Before torque can be transmitted again, a certain pressure must first be built up in the hydraulic system before the vehicle can start off. This results in a relatively long activation time until the gear is engaged following an engine start, which makes it practically impossible to have start-stop operation with a high frequency of starting processes and to meet the need, after starting the engine for example when the traffic lights change, to start off as promptly as possible since in practice delays which are too frequent and too long occur.
In order to enable start-stop operation, it is possible for example to use an additional pump driven by an electric motor which, when the combustion engine is switched off, maintains the oil pressure, or else a so-termed hydraulic impulse storage (HIS) system can be connected to the oil circuit of the transmission, by means of which the pressure build-up after an engine start is made substantially faster. However both of these, besides adding to the costs and weight, increase the energy consumption and therefore have a rather counterproductive effect on the energy balance of the start-stop operation, so that where possible they should not be used.
DE 10 2007 003 923 A1 discloses a method for controlling an automatic transmission of a motor vehicle, with hydraulically actuated shifting elements for the engagement of gears, having a hydraulic pump driven by a combustion engine for delivering pressurizing and cooling oil. To reduce an actuation time of the transmission after an engine start, it is provided that when the combustion engine is switched off for a short time, the automatic transmission is set to an electrical shift position of an engaged gear independently of any hydraulic supply pressure.
In DE 10 2007 003 924 A1 a hydraulic system of an automatic transmission is disclosed having a hydraulic pump driven by a combustion engine for supplying pressurizing and cooling oil to hydraulic shifting elements of the automatic transmission. When the combustion engine is switched off, in order to delay a pressure drop in the main pressure circuit, according to DE 10 2007 003 924 A1 it is proposed to connect the main pressure circuit by way of a connecting line containing a one-way valve to a retarder reservoir. The disadvantage of this, however, is that in the hydraulic system an additional connecting line and a one-way valve arranged in the connecting line are needed, which entail additional costs.